I just want to comment on the general idea that "dealing damage for tanks and healer is not mandatory" or that it's simply a secondary thing. (I'm not answering directly to NephthysVasudan because the whole post is a complete mess of contradictions making me impossible to know what "side" is taken here. I'm just quoting for the sake of having a reference.)
Your main objective as a team is to reduce the HP of the enemies you fight to zero. This is you main goal no matter your role. It's your goal as a Tank, a Healer or a DPS. All of you share that same objective. This means that your job as a party member is to do everything you can to reach that goal.
If you do not deal damage as a Tank or a Healer when you can, then you are absolutly not contributing like you should as a party member. You are letting other carry the burden that you could've carried yourself.
And that leads to the second reason why the argument of "it's optional" is completly hollow. If you have the option to contribute but aren't doing it because of subjective reasons in the context of a team effort, it simply means that you are a selfish jerk. You are dragging everyone down, when you could go up. If your own personal agenda is to shoot for the lowest possible level, then you should be playing singleplayer games. No exceptions.
In short: if you can do it, you should do it. It cannot be more simple.
The reason why the complains are on the forums and not directly ingame is simple: when you queue for your daily roulette or whatever, if you get a princess healer doing nothing you'll usually not say anything out of the fear of starting a drama with a "you don't pay my sub!" individual. Besides, if that player is Lv.70 and is still playing like that, it's easy to assume that nothing that you'll say will change anything. So, you suck it up and deal with the fact that you are doing a dungeon with a terrible player. There's also the case where you are simply playing with a newbie. It's just not worth the risk.
Whatever the case, it's faster to get it done that way than starting an argument mid-dungeon and having some votekicks thrown around the place. So, yeah... People ingame are more silent about it because it's the best way to handle it live. It doesn't mean that people agree with princess healers (or tanks, but "princess tanks" deal a minimum of damage without even knowing. They're just not effective at doing it). They just silently endure the pain of having to deal with one.